This invention involves the incorporation of a phosphorescent pigment into cementious materials used in construction, concrete restoration, surface decorations, wall and pool plasters, and tile settings materials such as mortars and grouts. These materials will luminesce after charging with electromagnetic radiation. The luminescence will persist after the charging source illumination has ceased. The duration, 8 to 12 hours after the cessation of the source radiation, and intensity of the luminescence will be great enough so that the material will glow or luminesce visibly in low light level conditions for either aesthetic effect and/or safety considerations. The incorporation of these pigments will in no way detract from the performance characteristics of the cementious materials primary function or application. The phosphorescence cementious materials will in no way present a health or environmental hazard.
Cement based compositions enjoy broad application in construction materials, tile setting, wall and pool plasters, stucco, self leveling compounds, roofing tiles and cement patches. The addition of dyes and pigments to the cementious materials has also enjoyed wide application in all of the above mentioned materials. Phosphorescence is a specific type of luminescence in which the emission of radiation resulting from excitation of a crystalline or liquid material occurs after the excitation has ceased, and may last from a fraction of a second to hours or more. The absorbed radiation moves electrons from the lowest state, the ground state, to higher energy states known as excited states. The absorbed radiation may be simple reradiated at the same frequency or energy may be lost to the solvent or lattice and radiated at a lower frequency. In the fluorescence process, the excited electron remains in the same spin state and the relaxation time, the time between absorption and emission of radiation, is very short, on the order of 10xe2x88x923 to 10xe2x88x928 sec. With phosphorescence, the excited electron undergoes a xe2x80x9cspin flipxe2x80x9d in a process referred to as intersystem crossing. This electron is now trapped in a system in which the rapid return to the ground state is xe2x80x9cforbiddenxe2x80x9d. The key distinction is that the emission of absorbed radiation in phosphorescence can continue long after the cessation of the excitation, or source, radiation. In fluorescence, the emission is very short lived after the source excitation has been terminated. Hence, luminescence in fluorescence requires that the excitation be employed at all times, whereas luminescence from phosphorescence requires the excitation source to xe2x80x9cchargexe2x80x9d the material and the luminescence can be detected for long periods of time after the source excitation has stopped. The two processes are distinct both in mechanism and characteristic.
The process of phosphorescence in many materials requires the presence of heavy atoms which can be toxic, radioactive or prohibitively expensive. A new class of phosphorescent materials, rare-earth doped calcium aluminates, present unique opportunities for this invention. The rare-earth""s represent nonradioactive heavy atoms with little toxicity. The key advantage of this class of phosphorescent materials is that undoped calcium aluminate is already a component of Portland cement. The content of the calcium aluminate various with the type of cement, mostly controlling the cure rate and early hardness of the cement. Calcium aluminate has very limited solubility in water and the replacement in the crystal of a rare earth metal cation for calcium further reduces the solubility. This substitution of the rare earth doped calcium aluminate for the regular calcium aluminate will result in now other change in the performance of the cement aside from the phosphorescence. The rare earth doped calcium aluminates are also stable and continue to exhibit phosphorescence in the alkaline conditions of wet cement. Many other materials phosphorescence is pH dependent. The conditions present in the preparation and application of cementious materials may well lead to partial or complete degradation of the phosphorescence with other materials.
Previous patents have described inventions similar to this proposal. In the first three related patents, fluorescence is the luminescence mechanism either by direct reference for the implication of constant source irradiation. U.S. Pat. No. 4,172,063 xe2x80x9cAbrasion resistant Reflective Marking Compositionxe2x80x9d limited it""s description to fluorescent pigments and in several places refers to the use of either head lights or black lights as an excitation sources, further reinforcing the characteristic properties of fluorescence and not phosphorescence. Also, the targeted application of the patent is in marking materials and only utilized cementious materials for their abrasion resistance and bonding ability. There is no discussion of the use of cementious materials as construction materials or coating with aesthetic appeal. U.S. Pat. No. 5,314,536 xe2x80x9cMethods of Making Luminous Construction Materialsxe2x80x9d focuses on the preparation of luminescent materials, that is embedding luminescent chunks into cementious materials. This patent also limits its description to fluorescence and repeatedly refers to the need for a UV excitation source in order to observe the luminescence. U.S. Pat. No. 5,849,218 xe2x80x9cFluorescent Pool Coatingxe2x80x9d again limits its invention to fluorescent pigment and makes no mention of phosphorescence.
U.S. Pat. No. 6,005,024 xe2x80x9cPhosphorescent Epoxy Overlayxe2x80x9d does directly refer to phosphorescence and cites the use of rare earth doped calcium aluminates. However, the targeted application is in transparent overly materials to be used in marking applications. It specifies that the epoxy should be clear or translucent. This specification is key to that invention""s use as an overlay material which would not cover existing surface markings or obscure the surface""s original appearance. The material into which the phosphorescent pigment is incorporated is epoxy based material and at no time mentions cementious materials, as this would be in direct conflict with the issue of transparency. In this invention, the material should not be transparent and will actually serve as the material from which the surface will derive its appearance. The base material for this invention are cementitous and are related to the construction and tile setting industries.
Other patents (U.S. Pat. Nos. 5,424,006 and 5,665,793) are even further removed as the luminescent material is incorporated into hydrocarbon based systems such as paints. The hydrophobic nature of hydrocarbons would prohibit the inclusion of phosphorescent paints into cementious materials which are prepared for application from their dry form with water.
This invention involves the use of rare earth doped calcium aluminates as a phosphorescent pigment incorporated into a cementious matrix. The advantage of phosphorescence over fluorescence is that the sun can serve as the excitation source and after the sun has set the pigmented material will luminesce for several hours at an intensity plainly visible to the eye. The luminescence emitted can serve either a decorative or safety function. Dependent upon the manufacturing process, luminescent spanning the visible spectrum from blue to red is possible. Due to the chemical identity of the phosphorescence pigment, it can be readily incorporated into cementious material with no deleterious effects to the original function of the cementious matrix as a construction material or decorative surface preparation. Due to its insolubility in water and inertness in cementious based materials such as tile mortars and grouts, pool plasters, wall plasters, cementious roofing materials, cementious surface coatings, cement patches and self leveling compounds, the rare earth doped calcium aluminates make a unique match as a additive. The glow in the dark characteristic of phosphorescent materials eliminates the need for excitation sources like UV lamps to be operating in order to detect the luminescence. The phosphorescent pigment can be recharged, repeated, with little or no degradation of the luminescence over time. Research has shown that the phosphorescent pigment is stable in the cementious matrix whether it is in a dry setting like a concrete surface coating or in an aqueous environment like a pool plaster. The phosphorescent pigment can be incorporated into the cementious matrix directly or coated on the silica aggregate commonly used as filler in cement based preparations. The pigment is also compatible as inert filler in polymeric materials such as epoxies, acrylic and polycarbonates which are used in the modification of cementious materials to increase set strength, flexibility, chemical resistance or reduce shrinkage. These pigments are also compatible with other agents incorporated into cementious preparations to effect rheology, pumpability, air entrainment, or the growth microbes. The presence of the pigment in these crementious materials has had no effect on how the material is applied whether by brush, trowl or spray. The intensity of the luminescence is dependent upon the amount of pigment used. Pigment loads from 1 to 30% of the cementious base have been successful in giving perceivable luminescence even in low light level environments. The appearance of these pigments under visible light is a free flowing white powder or may contain some coloration. The white powders blend in with the cementious material and there is no discernible difference in the appearance of the cementious materials under visible irradiation such as sunlight. Removal of the source excitation, the lights turned off indoors or the sun setting outdoors, and the cementious materials now glows in a variety of colors dependent on the rare earth doping of the calcium aluminate.
The following examples are to more fully demonstrate the invention. Minor variation of these compositions will be considered equivalent by those skilled in the art and are included within the scope of the invention.